1. Technical Field
Disclosure generally relates to a composition for film formation, an insulating film, a semiconductor device, and a process for producing the semiconductor device.
2. Background Art
Silica (SiO2) films formed by vacuum processes such as chemical vapor deposition (CVD) have hitherto been commonly used as interlayer insulating films, for example, in semiconductor devices. However, even for CVD-SiO2 films having the lowest relative permittivity (i.e. dielectric constant k) among inorganic material films, the dielectric constant is about 4. The dielectric constant of SiOF films, which have been developed as CVD films with low-permittivity (low-k), is about 3.3 to 3.5. The SiOF films, however, are highly hygroscopic, and the its dielectric constant disadvantageously increases over time of use.
In recent years, coating-type insulating films, composed mainly of a hydrolyzed-dehydrocondensation product of a tetraalkoxysilane (TEOS), called SOG (spin-on-glass) films are also being used with a view to forming more uniform interlayer insulating films. Here, the hydrolyzed-dehydrocondensation means a dehydrocondensation reaction between hydrolyzed products.
Further, further integration in semiconductor devices and the like has led to the development of low-k interlayer insulating films, composed mainly of a polyorganosiloxane, called organic SOG. In particular, low-k interlayer insulating films, composed mainly of polymethylsilsesquioxane (in which dice-shaped silicas of Si8O12 have been crosslinked to each other via an oxygen atom in each cross-linkage, a part of cross-linkages having been replaced with a methyl group), called MSQ have also been developed.
The organosilicon oxide films have two structural features. One of them is that a part of cross-linkages has been converted to a hydrocarbon termination such as Si—CH3, whereby the cross-linked structure is broken. The other feature is that a part of cross-linked structures by a Si—O—Si cross-linkage in silica has been replaced with cross-linked structures by a hydrocarbon cross-linkage (i.e. hydrocarbon cross-linked structure) such as Si—CH2—Si or Si—CH2—CH2—Si. Breakage of the cross-linked structure results in the formation of cross-linkage-free gaps, that is, vacant spaces or pores having a dielectric constant of 1, in the insulating film. As a result, the dielectric constant of the whole film is decreased. Further, an increase in the proportion of the hydrocarbon cross-linked structure to the Si—O—Si cross-linkage also leads to an increase in the volume proportion of the cross-linkage-free gaps in the insulating film.
An increase in the proportion of cross-linkage-free gaps (pores), however, poses a problem that spaces, which are formed in a production process of semiconductor devices and the like, act as disadvantageous sites which absorb and hold various substances, that is, particularly moisture and an etching gases, from the outside of the films.
Under such circumstances, a method is known in which a material comprising an organopolysiloxane and high-boiling point solvents or thermally decomposable compounds added to the organopolysiloxane is used as an insulating film material having excellent insulating properties, heat resistance, and mechanical strength in order to form pores in the film and thus to lower the dielectric constant of the film.
The above porous film, however, suffers from problems, for example, that the pore formation lowers the dielectric constant but, at the same time, weakens the mechanical strength and, further, an increase in dielectric constant occurs disadvantageously due to moisture absorption into the pores. Further, since pores connected to each other are formed, copper atoms used for interconnections disadvantageously diffuse through the insulating film. That is, it is required that the pores should be effective for reducing the dielectric constant of the insulating film but each of them has a small pore volume and are not connected to each other.
However, the organosilicon oxide films, particularly when the thickness is large (about 1 mm) as that of the interlayer insulating film like the upper layers in the multilevel interconnect technology, cause cracking disadvantageously due to the intrinsic stress of the films per se and deteriorate resistance against CMP (chemical mechanical polishing). It has been confirmed as the stress corrosion cracking that the occurrence and progress of this cracking are accelerated by moisture introduced into the film by moisture absorption. However, it is very difficult to control inexpensively for keeping an external moisture content on a low level in all of semiconductor device manufacturing process environments after formation of low-k insulating film.
In order to solve the above problems, techniques for producing insulating films have been developed in which a compound produced by cross-linking Si atoms with straight-chain alkyl (normal alkyl) groups is used with a view to lowering the dielectric constant and improving the mechanical strength. These techniques, however, have never achieved enough the requirements for dielectric constant, mechanical strength and moisture absorption resistance of the films.
Further, a technique in which an organosilicon oxide film using a compound cross-linked by an aromatic or other alicyclic group is used, or a technique in which an organosilicon oxide film using an aromatic or other alicyclic group is used in a part of a cross-linkage has been developed (see, for example, JP-A 2006-241304 (KOKAI). Furthermore, a technique in which a disilacyclobutane structure (Si(—C—)2Si) is used in a part of a cross-linkage has been developed (see, for example, JP-A 2006-111738 (KOKAI)). Even these techniques could not have satisfactorily met the requirements for dielectric constant, mechanical strength, and moisture absorption resistance of the films.
In these techniques, the number of carbon atoms contained in a cross-linkage moiety between silicon atoms, i.e., Si . . . Si, is disclosed to be from 1 to 50 in normal alkyl groups and is from 5 to 40 (preferably from 5 to 13) in a cyclic groups. Specifying the number of carbon atoms aims mainly at lowering the dielectric constant. The reason for this is as follows. Approximately two-thirds of the dielectric constant attributable to a siloxane skeleton of silica (SiO2) is governed by oxygen (polarization of oxygen ion). Oxygen also functions to hold the film density on substantially the same level as that of polymorphic form of SiO2 crystals by cross-linking between silicon (Si) atoms through oxygen. Cross-linking through carbon (I.e. hydrocarbons) is advantageous in that replacement of the oxygen with carbon can reduce the film density and can lower the polarizability of the film (both the electronic polarizability and the ionic polarizability).
However, material design guidelines based on clear-cut reasons for selecting the suitable number of carbon atoms in the cross-linkage moiety within the range from 1 to about 50, more specifically selecting a chemical structure (that is, cross-linkage structure) of these carbon atoms, and selection of materials according to the material design guidelines are not disclosed.
Of course, some precedent studies have been already reported on this guideline in IEEE International Interconnect Technology Conference, pp. 122-124, 2006 (reference 1) and Jpn. J. Appl. Phys., Vol. 46, No. 9A, pp. 5970-5974, 2007 (reference 2). In these references, a difference in potential energy curves for characteristic distortions (deformation potential curves) against external force applied to a cross-linkage moiety in interest is examined by computational simulations for cross-linkages by polymethylene groups in which the number of carbon atoms in the straight-chain alkyl (normal alkyl) group is in the range from 1 to 4, that is, four cross-linkage forms of CH2 cross-linkage (methylene cross-linkage), CH2CH2 cross-linkage (ethylene cross-linkage), CH2CH2CH2 cross-linkage (n-propylene cross-linkage), and CH2CH2CH2CH2 cross-linkage (n-butane cross-linkage). The results show two points that the dielectric constant decreases with increasing the number of CH2 cross-linkage units, but on the other hand, the mechanical strength (Young's moduli) (i.e. the hardness) increases with decreasing the number of CH2 cross-linkage units (i.e. decreasing the overall length of CH2 cross-linkages). Consequently, the references have concluded that, due to the contradictory relationship between both properties with respect to the number of CH2 cross-linkage units, CH2CH2 cross-linkage (ethylene cross-linkage) is the most suitable. According to the references, the reason why the mechanical strength increases with decreasing the number of (CH2)n cross-linkage units is that the effect of restricting the deformation of relative positions between two Si atoms located at both ends of the cross-linkage is improved with decreasing the number of (CH2)n cross-linkage units. That is, when the number of (CH2)n cross-linkage units is smaller (i.e. the length of the overall length of (CH2)n cross-linkage is shorter), the internal distortion energy is increased more rapidly against externally applied deformations, that is, the hardness is increased. However, the material design guidelines for the purposes of mechanical strength improvement in these references is entirely directed to an improvement in the skeleton strength of the film (an increase in Young's modulus).
In summary, the main purpose of conventional cross-linkage by a hydrocarbon group R (for example, a polymethylene group or a phenylene group) is to increase the hardness of the Si—R—Si skeleton (i.e., to increase tensile strength in a main chain direction, bending strength in a direction perpendicular to the main chain, or torsional strength around the main chain). The guiding principles in order to attain the purpose is that either the cross-linkage where the number of carbon atoms in Si—(C)n—Si cross-linkage is increased or the cross-linkage where a planar structure of the benzene ring is utilized because of Si—C—Si cross-linkage being stiffer than Si—O—Si cross-linkage. Cross-linkages by carbon (C) are likely to be fixed against bending in a direction perpendicular to the main chain or torsion around the main chain because of the nature of either sp3- or sp2-hybrid orbitals of C. On the other hand, in cross-linkage by oxygen (O), the Si—O bond per se is also strong. Since, however, the O atom ordinarily has twofold coordination, the degree of freedom of rotation and torsional deformation around O is high enough that the O cross-linkages appear to be “flexible” ones.
Further, as described above, increasing the proportion of the hydrocarbon cross-linkages to the Si—O—Si cross-linkages leads to an disadvantageous increase in cross-linkage-free gaps (pores) and thus results in deteriorating moisture absorption resistance. In addition, an organosilicon oxide film having a high hydrocarbon content is similar, in a sense, to a resist material. Therefore, as the carbon content increases, resistance against reactive ion etching (RIE) for resist patterning processes and to resist ashing treatment (ashing) for resist removing processes is disadvantageously deteriorated.
To overcome the above problems, studies have also been made to apply plasma treatment using N- or H-containing gas instead of the conventional oxygen gas-containing RIE or ashing process. In this case, carbon is removed, for example, as HCN. The use of such gases, however, also suffers from the problem that a higher carbon content causes a more significant deterioration in resistance against plasma treatment.